The present invention relates to a learning method for an automatic transmission of a vehicle and a system thereof, and more particularly, to a learning method and system for an automatic transmission of a vehicle when a down-shift is performed during an up-shift.
A vehicle automatic transmission performs shifting to a target shift-speed by controlling a plurality of solenoid valves that in turn control hydraulic pressure based on a variety of factors related to vehicle driving, such as a vehicle speed and throttle valve open-angle. For instance, when a driver trans-positions a select lever into a desired shift range, a manual valve transforms its port configuration, and therefore hydraulic pressure from a hydraulic pump is delivered to corresponding friction elements under control of the solenoid valves.
In the process of controlling shifting of the shift-speed, there is an element to be released, which is originally engaged, and there is also an element to be engaged (referred to as xe2x80x9cengaging elementxe2x80x9d hereinafter), which is originally disengaged. The timing for engaging and disengaging elements to be engaged and disengaged is important to enhance shift quality in an automatic transmission, therefore, much of the recent progress in shift control methods relates to providing proper engaging and disengaging timing of friction elements. The engaging and disengaging timing can be modulated by control of hydraulic pressure supplied to the friction element, and the hydraulic pressure is controlled by a hydraulic duty applied to the solenoid valves.
When the driver releases the accelerator pedal, a transmission control unit (referred to as xe2x80x9cTCUxe2x80x9d hereinafter) determines that a higher shift-speed is preferable, and accordingly an up-shift of the automatic transmission will occur, which is called a lift-foot-up shift. For example, when the accelerator pedal is released while the vehicle is being driven in a third shift-speed, with the select lever being disposed in a drive xe2x80x9cDxe2x80x9d range, the TCU starts a shift from the third shift-speed to a fourth shift-speed. However, if the accelerator pedal is depressed again before the up-shift is completed, the TCU must perform a down-shift to either the third shift-speed or a lower one.
When a down-shift is necessary when an up-shift has not been completed, the prior art usually performs the down-shift only after the up-shift is completed. This lengthens the period of time elapsed for shifting and therefore deteriorates acceleration of the vehicle. Further, even when the prior art starts a down-shift control while the up-shift is not complete, the prior art does not provide an appropriate learning method for the down-shift. This causes shift shock during the down-shift when clearance of the clutch has changed-due to abrasion or because of tolerance stack-up, and therefore deteriorates the durability of the automatic transmission.
The present invention provides a method and a system for learning a hydraulic duty for an engaging element of a down-shift, based on an amount of overrun incurred in the down-shift such that a minimal overrun is induced in the down-shift because of the learned hydraulic duty.
A learning system for an automatic transmission according to an embodiment of the invention comprises a revolution speed detector detecting at least one of engine revolution speed and turbine revolution speed; and a transmission control unit, wherein the transmission control unit performs a learning method according to the present invention.
A learning method for an automatic transmission according to a further embodiment of the invention comprises detecting an amount of overrun produced during a down-shift occurring in an up-shift; determining whether the amount of overrun is greater than a predetermined level; and modifying a hydraulic duty for an engaging element of the down-shift on the basis of the amount of overrun and a determination in whether the amount of overrun is greater than a predetermined level.
Preferably, modifying a hydraulic duty comprises performing one of adding or subtracting a correction value to the hydraulic duty, the correction value being calculated based on the amount of overrun. The correction value may be calculated as increasing as the difference between a amount of overrun and the predetermined level increases. More preferably, the correction value is added to the hydraulic duty when the amount of overrun is greater than the predetermined level, and subtracted from the hydraulic duty when the amount of overrun is less than the predetermined level. The predetermined level is preferably defined as a least amount of overrun such that a tie-up shock is not incurred.